This invention relates to a cathode ray tube, more particularly to the phosphor screen thereof.
In general, a color cathode ray tube is provided with a phosphor screen constituted by a stripe- or dot-arranged phosphor pattern emitting red, green and blue respectively, which is coated on the inside of an envelope panel. The screen is excited to emit the light by three electron beams generated from an electron gun.
In order to improve the contrast of the picture images on the screen, it has been known to reduce the reflectivity of the phosphor screen by means of pigmented phosphor.
As a green emitting phosphor for the color cathode ray tube, zinc type phosphors, e.g. ZnS:CuAl, (ZnS:CuAl--ZnS:AuAl), ZnS:CuAuAl, (Zn,Cd)S:CuAl are conventionally used. These sulfide phosphors have a relatively high efficiency to give a high brightness to the tube. The phosphors emit light by a subtle electron excitation as scattering electrons. Thus, such emission raises the background brightness of the picture, so the contrast ratio deteriorates. The use of a pigmented phosphor is one of the methods for improving the contract. The pigmented phosphor is a mixture of phosphor particles and colored pigment particles.
In the Japanese Patent Publication No. 58-27832, chromium oxide green, cobalt green and TiO.sub.2 --ZnO--CoO--NiO are disclosed as suitable green pigments. The role of the pigment is to reduce the reflectivity of the phosphor surface, while not reducing the background brightness caused by scattering electrons.
This is believed to be because these pigment particles with relatively small sizes are physically attached on the surface of the phosphor particles. The pigments cause a lowering of the phosphor luminescence efficiency, which is the ratio of an anode applied voltage to the value of brightness. In addition, the pigments cannot selectively prevent scattering electrons with various potentials from exciting the phosphor. This applicant has filed the Japanese applications (Japanese Patent Disclosure Nos. 60-156787, 60-199091 and 60-199092) disclosing methods of improving the degradation of picture image contrast caused by scattering electrons. These methods include doping a small amount of nickel, cobalt and iron ions into the green emitting zinc sulfide phospor particle surface as a killer to improve the contrast.
In particular, cobalt doping controls the voltage-brightness characteristics of the phosphor and nickel doping controls the current-brightness characteristics thereof. The above-mentioned disclosures describe reducing the effect of scattering electrons using both characteristics. Table 1 shows the characteristics of ZnS:CuAl phosphors with no doped, Co doped, Ni doped and Co--Ni double doped, respectively.
The double doped phosphor is made by firing with Co: 10 ppm and Ni: 0.75 ppm at the same time at 700.degree. C. for 45 min.
As shown Table 1, the green monochromatic brightness and background brightness of a phosphor screen assembled in a 20 inch type color cathode ray tube were measured. The screen using the double doped green phosphor, as compared with the other phosphors, showed a synergistic effect from Co and Ni doping in relation to the background brightness, but not to the green brightness. The green brightness tended to reduce unpractically.
TABLE 1 ______________________________________ green brightness background (relative value) brightness phosphor (beam current 1.6 .mu.A/cm.sup.2) (relative value) ______________________________________ no doped 100 100 Co:10 ppm 95 85 doped Ni:0.75 ppm 100 90 doped Co,Ni 88 76 double doped ______________________________________
Since Ni ion is easily diffused into the ZnS inner core in the presence of Co ions during firing, the luminescence efficiency is lowered. In Table 1, the background brightness was measured at a region less than 30 mm from the edge of the white window pattern while the screen was covered by a black paper. An area with 30 cm.times.10 cm was displayed at the upper half of the screen with an anode voltage of 26.0 kv and total beam current of 500 .mu.A.